Production of dibasic acids from c-8 aromatic hydrocarbons



Jan. 16, 1962 A. c. MCKINNls ETAL 3,017,433

PRODUCTION OF DIBASIC ACIDS FROM C-8 AROMATIC HYDROCARBONS Filed May 15. .1954

Patentedv Jan. 16, 1952 PRODUCTION F DHBASIC ACDS FRM C-8 ARUMATIC HYDRGCAREONS Art C. McKinnis, Long Beach, and William L. Wasley,

Santa Ana, Calif., assignors to Union Dil Company of California, Los Angeles, Calif., a corporation of California Filed May 3, 1954, Ser. No. 427,154

11 Claims. (Cl. 26o-522) This invention relates to methods for treating mixtures of C-S aromatic hydrocarbons which contain substantial proportions of ethylbenzene and meta-xylene to obtain a substantially complete molar conversion thereof to a mixture of terephthalic acid and isophthalic acid, which mixture contains at least 40% of terephthalic acid. Generally stated, the process comprises subjecting the mixture of C-8 aromatic hydrocarbons to an optimum degree of alkylation under certain critical conditions, then oxidizing substantially all the side-chains of the alkylate to obtain a complete mixture of dibasic yand tribasic acids, and finally subjecting the tribasic acid to decarboxylation to convert the same to a dibasic acid mixture which may have approximately the same isomer distribution as the dibasic acids from the oxidation step. Critica-l aspects of the invention reside in carrying out the alkylation step in such manner, and only to such a degree as to:

(l) Substantially completely alkylate the ethylbenzene to form predominantly a para-dialkyl benzene,

(2) Substantially completely alkylate any ortho-xylene present in the mixture to form a 1,2,4-trialkyl benzene,

(3) Alkylate only sucient of the metaxylene originally present -as may be alkylated without alkylating any significant portion of the p-xylene, and to form a 1,2,4- trialkyl benzene,

(4) Substantially restrict the ralkylation so that few if any benzene rings are more than mono-alkylated, and

(5) Leave the major portion of the para-xylene unalkylated.

The alkylation step contemplated herein results in a partially alkylated product which, when oxidized and subsequently decarboxylated, will give the highest molepercent conversion to dibasic acids, and the highest ratio of terephthalic acid to isophthalic acid, and at the same time will require the least consumption of oxidizing agent.

It is an object of this invention to provide a means for ut-ilizing mixtures of C-S aromatic hydrocarbons which contain substantia-l proportions of ethylbenzene and metaxylene, and may also contain ortho-xylene and/or paraxylene, whereby maximum conversions to dibasic acids are obtained, and the dibasic acids will contain the maximum proportion of terephthalic acid. A further object is to obtain terephthalic acid and isophthalic acid from mixtures of C-S aromatic hydrocarbons while avoiding the necessity for separating the hydrocarbon isomers. A further o'bject is to yachieve the foregoing ends through an alkylation-oxidation-decarboxylation sequence which will require the minimum of alkylating agent and oxidizing agent, and a minimum of handling throughout; A broader object is to provide raw materials for the production of isophthalic and terephthalic acids which are more plentiful and economical than those heretofore employed. Still further objects include the provision of techniques which will reduce corrosivity, heat requirements and process equipment to practical minimum values. Other objects and advantages will be apparent to those skilled in the art from the more detailed description which follows:

The aromatic dicarboxylic acids are highly important industrial raw materials by virtue of their use in the manufacture of polymeric esters for resins and synthetic fibers, monomeric esters for plasticizers, solvents and the like. In the past, terephthalic acid `and isophthalic acid have commonly been manufactured by the oxidation of pure prara-xylene and meta-xylene respectively. Such processes suffer from the disadvantage that metaand para-xylenes are very difficult to separate. This diiculty may be appreciated from the fact that -98% pure paraxylene is currently about three to four times as expensive as lthe mixed isomeric xylenes, which also contain ethylbenzene. The present invention takes advantage of the fact that such chemical treatments as alkylat-ion and aqueous decarboxylation are sufiiciently economical to compete with the known xylene separation methods. It is therefore feasible to employ considerably more in the way of chemical processing when utilizing a cheap raw material such as mixed xylenes, and still be competitive with processes requiring less chemical processing, but a more expensive raw material, i.e. para-xylene.

The cor-pending application of Art C. McKinnis, Serial No. 410,430, filed February 15, 1954, now U.S. Patent No. 2,734,914, describes a process whereby trimellitic acid (l,2A-benzene-tricarboxylic acid) may be cheaply decarboxylated by heating with water to form a mixture consisting of about 40% terephthalic acid and 60% isophthalic acid. in that application, the ralkylatio-n of mixed xylenes, and the subsequent oxidation thereof to form trirnellitic acid and terephthalic acid was Ialso described. However, it was suggested therein that the alkylation should be carried to the extent of forming only two geometric types of molecules i.e. 1,2,4-trialkyl benzenes and 1,4-dialkyl benzenes. It has now been discovered that alkylation carried to such an extentis unnecessary as applied to utilization of the mixed alkylate for conversion to dibasic acids through oxidation and thermal-aqueous deearboxylation. Exhaustive yalkylation is disadvantageorus both from the standpoint of requiring excessive alkylating agent, and excessive oxidizing agent. It is also disadvantageous because frequently the alkylation cannot be carried to the desired degree without `also producing higher alkylated benzenes i.e. di-alkylated and tri-alkylated xylenes. According to the present process no more of the meta-xylene is alkylated than can be alkylated without beginning to alkylate the p-xylene, which is the last isomer to be attacked. The formation of predominantly para-isomers from ethylbenzene insures that the final ratio of parato meta-dialkyl benzenes in the alkylate is rat least as high as the ratio of parato meta-acids which is obtained by the aqueous decarboxylation of the trimellitic acid. There are in fact several fortuitous relationships existing between the alkylation mechanics, i.e. the inherent distribution of alkylate isomers, and the particular decarboxy-lation procedure employed herein.

These fortuitous relationships may be more obvious froma consideration of the fate of each individual C-8 isomer in the process. Under the conditions described, it is found that ethylbenzene is converted predominantly, i.e. about 60-80 mole-percent, to a para-alkylated product, which is finally converted to terephthalic acid as follows:

It is also found that the ethylbenzene is the most easily alkylate-d o-f the isomers initially present. This is fortuitous because ethylbenzene is the least desirable isomer for oxidation-decarboxylation, inasmuch as it would terminate as benzoic acid.

The ortho-xylene is also an undesirable isomer in that it would terminate as ortho-phthalic acid, the least valuable of the dibasic acids. However, ortho-xylene is the next most readily alkylatabe isomer, and is mono-alkylated to form almost exclusively the 4-alky1 ortho-xylene,

COOH

COOH

COOH

COOH

A COOH COOH C O OH A certain amount of meta-xylene is tolerable in the alkylate. It is preferable however that its ratio therein to the remaining para-compounds does not exceed the ratio of metato para-acids which trimellitic acid yields by decarboxylation. This facilitates the combined separation of the final products, as will be described hereinafter. Meta-xylene is more difficult to alkylate than ortho-xylene, and hence the lat-ter may be substantially completely alkylated before all the meta-xylene is alkylated. It is therefore feasible to alkylate substantially all the ethylbenzene and the ortho-xylene, and to maintain some control over the proportion of the meta-xylene which is alkylated. The meta-xylene is in effect a buffer material, permitting the complete alkylation of o-xylene and ethylbenzene without alkylating the p-xylene. It is found also that meta-xylene is alkylated to form almost exclusively the 4-alkyl meta-xylene, which is ultimately converted to isophthalic and terephthalic acids as follows:

CH3 CH3 COOH CH3 cat. CH3 C O OH COOH COOH

COOH

H2O COOH C O OH Another fortuitous relationship accruing herein lies in the inherent difficulty of alkylating the para-xylene, or the para-alkylated ethylbenzene which is formed in the mixture. Para-xylene may be slowly alkylated to some extent, and increasingly as it becomes the most concentrated alkylatable lcomponent of the reaction mixture. Hence the desirability of maintaining some m-xylene diluent, or buffer. |The para-dialkyl benzenes derived from ethylbenzene -are even more difficult to alkylate than para-xylene due to the stearic effects of the larger alkyl groups. These para isomers give terephthalic acid directly upon oxidation. If they were alkylated to give trimellitic acid precursors, there would be a waste of alkylating agent and oxidizing agent, more extensive decarboxylation would be required, and about 60 molepercent thereof would be finally converted to the less valuable isophthalic acid.

Summing up these fortuitous relationships, it may be said in general that the order of alkylatability of the isomers coincides very advantageously with the order of desirability for their alkylation, when the total alkylate is to be treated as herein described. This enables us to utilize a very cheap raw material, and to produce large proportions of valuable products therefrom, with a minimum of alkylation, and the use of little more of oxidizing agent than is required for oxidizing para-xylene. It will be apparent also that no separation steps are necessarily involved until the final decarboxylated product is obtained. Furthermore, as will be more fully explained hereinafter, the particular decarboxylation step is interrelated with an economical method for effecting final separation of isophthalic and terephthalic acids.

The feed materials which may be utilized herein include in general any mixture of the C8 aromatic hydrocarbons which contains more than about l0 volume-percent, and preferably more than l5 volume percent of ethylbenzene, and more than about 20%, or preferably more than 30%, of meta-xylene. A further preference is 4that the feed mixture should contain between about 5% and 25% by volume of para-xylene, and about 10% or more of o-xylene. If these requirements are observed, it will be found that regardless of the over-all ratio of isomers in the mixture, it may be treated as disclosed herein to yield a final mixture of dibasic acids wherein the proportion of terephthalic acid is higher than that which would be obtained by the decarboxylation of pure trimellitic acid, i.e. more than 40 mole percent. The adapatability of the process to feeds of such varying compositions constitutes one of the principal advantages. It is applicable for example to the equilibrium mixture of xylenes which may be obtained from reformed gasolines, preferably naphthenic gasolines. Such reformed gasolines ordinarily contain from about 40-60 volume percent of aromatic hydrocarbons which may be easily separated from the non-aromatics by extraction with e.g. diethylene glycol, thiodipropionitrile, oxydipropionate, sulfur dioxide, or any of the well known selective solvents for aromatics. The C-8 hydrocarbons contained in the aromatic extract boil at about 13G-144 C., and may be isolated therefrom by fractionation.

Instead of separating the total aromatic content from the gasoline and then fractionating to obtain pure C-8 hydrocarbons, the gasoline may be first fractionated to recover a cut boiling at e.g. 13S-145 C., which may then be treated to separate the aromatics from the nonarornatics. This separation may be achieved by solvent extraction, or by azeotropic distillation with e.g. methylethyl ketone, nitromethane, or any other material which is capable of azeotroping the parafns overhead. However, any other source of pure or impure xylenes may be employed.

The alkylating agent employed herein may be any of the known alkylating agents containing from 2 to 4 carbon atoms which are capable of being introduced onto the benzene nucleus under conditions which do not also effect isomerization. Suitable alkylating agents include for example ethylene, propylene, butylene, isobutylene, isopropyl alcohol, ethanol, isopropyl chloride, ethyl chloride, and the like. The preferred alkylating agents comprise the olens containing from 2 to 4 carbon atoms, and propylene appears to be the most effective of these. The known methylating agents are apparently ineffective for the present purposes because known methods for their introduction onto the benzene ring require isomerizing conditions.

Reference is now made to the accompanying drawing which illustrates schematically some of the process features of the invention. Certain critical features of the invention will be described in reference to the illustration, but the illustration should not be regarded as limiting in scope.

In the modification illustrated, the initial alkylation is conducted in a tubular vessel or column 1, which is packed with a suitable alkylation catalyst 2. This particular apparatus is designed for liquid phase propylation with propylene wherein the catalyst is in the form of stationary beds of granular material. The preferred catalyst for the alkylation consists of phosphorus pentoxide, either in the form of lumps, or in the form of a nely powdered material deposited on a granular supporting material such as charcoal, silica gel, pumice, acid-activated clays and the like. Such catalysts may be prepared by agitating powdered phosphorus pentoxide under anhydrous conditions with the carrier. Granular carriers ranging between about and 50 mesh may suitably be employed. Suitable ratios of phosphorus pentoxide to carrier may range between about l/ 1 and 1/ 50 by weight. The carrier may be suitably ground or pelleted to meet the desired iiow conditions in the reactor.

The phosphorus pentoxide employed herein may be. modified by mixing therewith certain proportions of various acidic promoters such as p-toluene sulfonic acid, arsenic trioxide, orthophthalic acid, phosphoric acid and the like.

The use of phosphorus pentoxide catalysts is prefeired because they appear to exhibit a specific action favoring the formation of para-alkylated products. They also meet the more general requirements for alkylating catalysts used herein in being non-isomerizing and nonpolymerizing. In general any other alkylation catalysts may be employed which are non-isomerizing and nonpolymerizing under the alkylation conditions. Such other catalysts include for example sulfuric acid and phosphoric acid. These particular catalysts do not exhibit the same selectivity in the formation of para-isomer as does phosphorus pentoxide, but do favor the formation of 1,2,4-trialkyl benzenes. They may therefore be employed to slightly less advantage than phosphorus pentoxide.

Alkylating conditions in reaction vessel 1 include maintaining temperatures between about 50 and 140 C. and preferably between about '75 and 130 C. Pressures are preferably maintained at about atmospheric in order to avoid polymerization of propylene. However, reduced pressures or slightly super-atmospheric pressures may be employed if desired.

The liquid feed material consisting of ethylbenzene plus xylenes is admitted to the top of column 1 through line 3 and tiows downwardly therethrough countercurrently to gaseous propylene admitted to the bottom of the column through line 4. Any unabsorbed propylene is taken off at the top of the column through line 5 and passed through a condenser 6 to redux any vaporized xylenes downwardly. Temperatures in the alkylation vessel may be controlled by preheating the reactants, or by means of internal heat exchangers, such as heating coil 8.

A particular feature of the invention resides in avoiding the introduction of more than one alkyl group per molecule. This may be accomplished by withdrawing all or a portion of the reaction mixture at an intermediate point or points during the alkylation, separating the unreacted xylenes from the alkylated portion, and returning the non-alkylated xylenes to the reactor. Preferably, the concentration of alkylated hydrocarbons in the mixture should be maintained at less than about 25 mole-percent by such means. This feature is illustrated herein by a single Withdrawal of intermediate product through line 9, which is then transferred to distillation column 10 to fractionate overhead unreacted xylenes .which are then recycled via line 11 to a point in the alkylation column adjacent to the point of withdrawal. A baiiie plate 13 is provided between the withdrawal and inlet ports in order to prevent backflow. The bottomsv from distillation column 10 is taken off through line 14 and mingled with the final alkylate, as will be apparent hereinafter.

Under the conditions described, the degree of alkylation may be readily controlled by merely adjusting the flow rates of liquid feed material, or propylene, or by varying the temperature. Any or all of these factors 6 may be varied in response to variations in the ratio of alkylated to non-alkylated xylenes in the iinal alkylate mixture. This final alkylate is formed by mingling the bottoms product from line 14 with the secondary alkylate withdrawn near the bottom of the column 1 through line 15. This combined alkylate is then transferred through line 16 to the oxidation step indicated at 17.

The oxidation step may be carried out by any of the procedures which are known in the art. The oxidation of side-chain alkyl groups to form carboxyl groups is well known, and the present objective is to substantially completely oxidize all the side-chain alkyl groups. This may be accomplished for example by oxidation in the liquid phase with '3C-40% nitric acid at temperatures between about and 300 C., either with or without add-ed air or oxygen. Alternatively, an initial oxidation may be carried out with air in the presence of for example 1% of a cobalt naphthenate catalyst, or other group VIII metal salt at 1D0-175 C. for three hours, and the resulting mono-carboxylic acids may then be further oxidized with nitric acid at -250 C. in the absence of catalyst. Alternatively, the hydrocarbons may first be oxidized with air as described, and the oxidation of the monocarboxylic acids completed by heating with sodium bisullite and hydrogen sulfide in the presence of water at a temperature between about 500 and 700 F. The latter method of oxidation is more particularly described in the copending application of Art C. McKinnis, Serial No. 377,399, filed August 31, 1953, now U.S. Patent No. 2,809,204. Any other suitable oxidation procedure may be employed.

The product from the oxidation step, as vrecovered through line 18, consists mainly of trimellitic acid, terephthalic acid and isophthalic acid. Very small amounts of ortho-phthalic acid may be present as a result of traces of unre-acted ortho-xylene, or ortho-alkylated ethylbenzene. Inasmuch as trimellitic acid is very solubile in water while terephthalic and isophthalic acids are much less soluble, it is possible to effect a simple separation at this stage by water extraction. However, it is preferred to do this only in cases where the mixture contains a large proportion of isophthalic acid in relation to trimellitic acid. The decarboxylation step described herein does not affect terephthalic or isophthalic acids, and either or both of those materials may therefore be carried through that step, along with trimellitic acid. This procedure has the advantage of requiring only one separation step.

In this preferred illustrative case, the entire oxidation product is transferred through line 20 to a mixing vessel 21 to which water is admitted through line 22., and to which a recycle liquor may or may not be admitted through line 23. This recycle liquor is mainly aqueous trimellitic acid derived as hereinafter described. In any event, suiiicient water is added to mixer 21 to dissolve, at the decarboxylation temperature, all of the isophthalic acid which was initially present, and all the isophthalic acid which is produced by decarboxylation of the trimellitic acid. This mixture is then stirred into a slurry and pumped by means of positive displacement pump 24 4through line 25, heater 26 and into a pressure vessel 27. The mixture in vessel 27 is maintained, by means of heater 26 and/ or heater 2S, at a temperature of between about 150 and 400 C., and preferably between about 220 and 350 C., until the evolution of CO2 substantially ceases. The CO2 evolved by decarboxylation is exhausted through line 29 and pressure relief valve 30, The mono-decarboxylation of trimellitic acid under these conditions is usually substantially complete in about 15 minutes to three hours, depending upon the temperature. Under the stated temperature conditions the pressure in the reaction vessel will vary between about 15 tok 190 atmospheres, providing that the CO2 formed is continuously exhausted.

The decarboxylation carried out in vessel 27 is a specitic type of decarboxylation which is critical tothe combination of procedures herein described. It is essential that the decarboxylation be conducted at the prescribed temperatures in the presence of a substantially neutral aqueous reagent. The initial pH of this reagent, before admixture with the acids to be treated, should preterably be between about and 9. If the initial reagent is more acidic, i.e. if an extraneous soluble acid is added thereto, it is found that the decarboxylation is retarded, and substantially no decarboxylation will occur under the stated reaction conditions. The preferred reagent is ordinary tap water, or distilled water, with no other additions. However, in some cases, as when low temperatures are employed, it may be desirable to add Lcertain neutral metal salt accelerators, such for example as soluble copper salts or manganese salts.

The proportion of water employed relative to the trimellitic acid is not critical to the decarboxylation process itself. Any amount of water is operative to some extent, but it is found that the larger the ratio of water to trimellitic acid, the more rapid will be the decarboxylation. It is therefore preferred to employ at least about 500 ml. of water per mole of trimellitic acid in order to accelerate the reaction.

If the preferred separation procedure is employed, it may be desirable to employ even more water than is necessary to obtain rapid reaction. For this purpose suiiicient water should be employed so that when 60 mole-percent of the trimellitic acid is converted to isophthalic acid, that amount of isophthalic acid, plus any initially present, will still remain in solution at the temperature which is to be subsequently employed for separating terephthalic acid. The amounts to be employed for obtaining these objectives will be apparent from the following table which shows the approximate solubilities of isophthalic acid and terephthalic acid in water at various temperatures. At each temperature listed, the corresponding amount of trimellitic acid which will' produce a saturated isophthalic acid solution at that temperature is also listed:

TABLE 1 Solubility, gms/100 ml. H2O

Temp., C.

isophthalic terephthalic aci 'd 2. 6 O. 3 (a) 10 (u) 1. 1

a Interpolated or extrapolated data.

b Grams of trimellitic acid per 100 ml. water which would yield saturtltei :isophthalic acid solution, assuming 60 mole-percent; conversion to 1; e a er.

It will be apparent from the above table that, at temperatures within the decarboxylation range (220-400 C.), the solvent capacity of water for isophthalic acid becomes significant, while the terephthalic acid is still largely insoluble. This relationship permits anA effective separation of the two acids by fractional crystallization from Water. The bulk of the terephthalic acid may be recovered by iiltration at or near the temperature of decarboxylation, and the bulk of the isophthalic acid by filtration at a lower` temperature. This separation may be achieved in conjunction with either batch or continuous decarboxylation.

In some cases it may be undesirable to employ in the decarboxylation the large volume of water which is required to keep the isophthalic acid dissolved while separating the terephthalic acid. In such cases, especially where the original oxidate contained a large proportion of isophthalic acid, the decarboxylation may be carried only to the extent which will saturate the solution with isophthalic acid at the terephthalic acid separation temperature. This involves a recycling of unreacted trig mellitic acid solution. Any of these modiiications may be carried out in the illustrated apparatus.

ln all cases the decarboxylated mixture is withdrawn through line 32 at a rate regulated by valve 33. Under autogenous pressure the mixture is passed through an optional cooler 34, and line 3S, into a first pressure iiltration unit 36. Filtration is carried out therein at a teniperature between about 250 C. and the decarboxylation temperature, preferably from 270-350 C. Substantially pure terephthalic acid collects in the bottom of iilter 36, and is periodically removed through valved line 37.

The mother liquor from iilter 36 is taken oft through line 39, cooled in exchanger 40, and passed via line 41 to a second tiltration unit 42 wherein isophthalic acid is separated. Filtration in this unit is conducted at a substantially lower temperature than in filter 36. Suitable temperatures may range between about 20-l50 C. for example. isophthalic acid is periodically removed through valved line 43.

The mother liquor from filter 4Z is taken off through line 45. If it still contains substantial proportions of trimellitic acid, the bulk thereof may be recycled via line 46 to mixing vessel 21. In this case it is generally necessary to remove a small slip-stream, e.g. 10% thereof, through line 47 to prevent the build-up of other impurities in the system, such as o-phthalic acid or benzoic acid. lf the decarboxylation in vessel 27 is carried to completion, all of the mother liquor from iilter 42 is exhausted through line 47 to sewer, or for recovery of any remaining values.

As an alternative to the procedure wherein the total oxidate is carried through the decarboxylation step, it may be desirable, when the oxidate is rich in isophthalic acid, to eifect a rough separation of water-soluble from water-insoluble acids7 and subject only the water-soluble portion to decarboxylation. In this case the oxidate from line 18 is slurried with sufcient water at e.g. 25-l 00 C. to dissolve the trimellitic acid, and the mixture is then allowed to settle in a settling zone indicated at 19. The insoluble dibasic acids which settle out are then drawn oic through line 50, preferably as a heavy slurry, from the lower part of the settling zone. Additional water is then added to the slurry through line 51, and the mixture is heated in heater 52 to a temperature sufticient to dissolve the isophthalic acid, but not the terephthalic acid. The hot mixture at e.g. 270-350 C. is then pumped via line 53, pump 54, and line 55 into line 35 carrying the trimellitic acid decarboxylate to irst ltration unit 36, wherein terephthalic acid is separated as previously described. This procedure ordinarily offers no particular advantage unless the oxidate from line 20 contains a high ratio of isophthalic acid to trimellitic acid, in which case the large amounts of water required to dissolve the isophthalic acid would unduly limit the volume capacity of the decarboxylation vessel. Where the oxidate contains not more than about 1.5 moles of isophthalic acid per mole of trimellitic acid, it is preferred to pass the whole mixture to the decarboxylation step.

The following example is given by way of illustration, and should not be considered as limiting:

Example I A glass column 3 inches in diameter, 3.5 feet in length, and equipped with a steam jacket and a reiinx condenser is selected for the alkylation reactor. This column is then packed with about 250 ml. of alkylation catalyst consisting of 50 parts by weight of 12-28 mesh charcoal and 25 parts of powdered phosphorus pentoxide. A feed mixture consisting of (by volume) 16 parts p-xylene, 19 parts ethylbenzene, 45 parts m-xylene and 20 parts of 0-xylene is then passed downwardly through the column at the rate of rnl. per hour, while gaseous propylene is bubbled upwardly at the rate of 0.4 s.c.f. per hour. The

liquid residence time is about one hour, and the average column temperature is maintained at 11G-120 C.

After 2 hours of operation, the accumulated liquid product is distilled to recover unreacted xylenes, which consist principally of p-xylene and m-xylene, with a small proportion of o-xylene. These xylenes are then again passed through the column under the same alkylating conditions. The resulting secondary alkylate is found to contain about 90% of the original p-xylene, and 55% of the original m-xylene. It is then combined with the alkylated fraction from the first pass, and the mixture is oxidized under pressure in a stainless steel autoclave with 30% nitric acid at 190 C., while continuously bubbling oxygen therethrough. When the oxidation is complete, as evidenced by the substantial absence of CO2 in the off-gases from the autoclave, the mixture is cooled to 20 C. and ltered. Excess nitric acid is removed from the solid by washing with small amounts of cold water.

The entire solid oxidation product is then mixed with 3 liters of water and placed in a larger stainless-steel autoclave equipped with a gas pressure relief valve, and a valved liquid-withdrawal port in the lower section. The contents are heated at 325 C. for one-half hour, while continuously withdrawing CO2. Without releasing the pressure, the contents are then allowed to cool to about 270 C. After a 15 minute period of agitation at this temperature, the valve in the liquid-withdrawal port is opened and the contents are forced under pressure through an external enclosed filtration unit. The precipitate is washed with warm water, dried and weighed. About 140 gms. of 98% pure terephthalic acid is obtained, representing a 42.0 mole-percent conversion of the original hydrocarbons.

The mother liquor from the ltration is then cooled to room temperature (25 C.) and again filtered. The second precipitate is washed with 200 ml. of warm water, dried and weighed. About 165 grams of 89% pure isophthalic acid is obtained, the pure component representing a 43 mole-percent conversion of the original hydrocarbons.

Example Il The procedure of Example I is repeated, using as feed an aromatic C-8 fraction from a platinum-alumina reformed gasoline, from which most of the o-xylene has been removed. The feed composition by volume was 23% ethylbenzene, 47% meta-xylene, 25% para-xylene and o-xylene. The mole-percent conversion to terephthalic acid is 46%, and to isophthalic acid, 39%.

Example III The procedure of Example I is repeated, using as feed a mother liquor from a first-stage fractional crystallization of p-xylene. Its composition was 23.5% ethylbenzene, 24% o-xylene, 45% m-xylene, and 7.5% p-xylene. The mole-percent conversion to terephthalic acid is 38%, and to isophthalic acid 43.5%.

From the above examples, it is clear that the procedures herein described are remarkably versatile for treating various mixtures of C-S aromatic hydrocarbons. In nearly all cases, the total conversion to dibasic acids is 70-90 mole-percent, and the ratio of terephthalic to isophthalic acid always ranges between about 40/ 60 and 60/40.

While in the above examples and description, specific materials and conditions have been discussed, it is not intended that the invention should be limited to such. Many variations will be apparent to those skilled in the art, and it is intended to include such variations within the scope of the claims:

We claim:

1. A process for producing a mixed alkylate from a mixture of C-S aromatic hydrocarbons comprising by volume at least about 10% of ethylbenzene, at least about 20% of m-xylene, and a substantial proportion, over 5%,

of p-xylene, said mixed alkylate being particularly adapted for total conversion to terephthalic acid and isophthalic acid via oxidation and decarboxylation, which comprises subjecting said mixture to alkylation with propylene, in the presence of a catalyst selected from the class consisting of phosphorus pentoxide, phosphoric acid and sulfuric acid, said alkylation being effected at a temperature between about 50 and 140 C., continuing said alkylation for a time suicient to effect substantially complete monoalkylation of said ethylbenzene and any o-xylene present, and terminating said alkylation (1) before any substantial amount of p-xylene has been alkylated, (2) before any substantial amount of polyalkylation has occurred, and (3) before complete monoalkylation of said m-xylene has occurred.

2. A process as defined in claim 1 wherein said alltylation catalyst is essentially phosphorus pentoxide.

3. In the catalytic alkylation of C-8 aromatic hydrocarbon mixtures with propylene alkylating conditions including the use of temperatures between about 50 and C., and a catalyst selected from the class consisting of phosphorus pentoxide, phosphoric acid, and sulfuric acid, whereby alkylation of the C-8 isomers is favored in the decreasing order: ethylbenzene, o-xylene, m-xylene, and p-xylene, and where the resulting total alkylate is used for producing terephthalic acid and isophthalic acid via oxidation of alkyl side-chains to carboxyl groups, followed by neutral aqueous mono-decarboxylation of the trimellitic acid formed, the improvement which comprises using as feed to said alkylation a C-8 mixture comprising by volume at least about 10% of ethylbenzene, at least about 20% of m-xylene, `and a substantial proportion, over 5%, of p-xylene, and carrying said alkylation to the extent of effecting substantially complete mono-alkylation of ethylbenzene and o-xylene, and terminating said alkylation (l) before any substantial amount of p-xylene has been alkylated, (2) before any substantial amount of poly-alkylation has occurred, and (3) before complete mono-alkylation of said m-xylene has occurred.

4. A process as defined in claim 3 wherein said catalyst is essentially phosphorus pentoxide.

5. A process for producing terephthalic acid and isophthalic acid from a mixture of C-S aromatic hydrocarbons comprising by volume at least about 10% of ethylbenzene, at least about 20% of m-xylene, and a substantial proportion, over 5%, of p-xylene, which comprises subjecting said mixture to alkylation with propylene at 50 to 140 C. in the presence of an alkylation catalyst selected from the class consisting of phosphorus pentoxide, phosphoric acid, and sulfuric acid continuing said alkylation fcr a time sufficient to effect substantially complete mono-alkylation of said ethylbenzene and any o-xylene present, terminating said alkylation (1) before any substantial amount of p-xylene has been alkylated, (2) before any substantial amount of poly-alkylation has occurred, and (3) before complete mono-alkylation of said m-xylene has occurred, subjecting the resulting alkylation mixture to oxidation to convert substantially all of the alkyl side-chains to carboxyl groups thereby forming a mixture of dibasic acids and trimellitic acid, thereafter subjecting said trimellitic acid to decarboxylation by heating at between about and 400 C. in the presence of a substantially neutral aqueous reagent to produce a second mixture of dibasic acids consisting essentially of terephthalic acid and isophthalic acid, and recovering terephthalic acid and isophthalic acid from each of said two mixtures of dibasic acids.

6. A process as dened in claim 5 wherein said alkylation catalyst is essentially phosphorus pentoxide.

7. A process as defined in claim 5 wherein said mixture of C48 aromatic hydrocarbons also contains at least about 10% of o-xylene.

8. A process for producing terephthalic acid and isophthalic acid from a mixture of C-8 aromatic hydrocarbons comprising by volume at least about 10% of ethylbenzene, at least about 20% of m-xylene, and a substantial proportion, over of p-xylene,-which comprises subjecting said mixture to a first stage alkylation with propylene at 50, to 140 C. in the presence of an alkylation catalyst selected from the class consisting of phosphorus pentoxide phosphoric acid, and sulfuric acid, continuing said alkylation until not more than 25 molpercent of said hydrocarbons have been alkylated, separating the alkylated hydrocarbons from the non-alkylated hydrocarbons, further alkylating said non-alkylated hydrocarbons in a second stage under the conditions herein prescribed for said rst stage alkylation for a time sufcient to effect substantially complete mono-alkylation of ethylbenzene and o-xylene, terminating said secondstage alkylation (1) before any substantial amount of p-xylene has been alkylated, (2) before any substantial amount of poly-alkylation has occurred, and (3) before complete mono-alkylation of said m-xylene has occurred, subjecting the combined alkylation mixture to oxidation to convert substantially all the alkyl side-chains to carboxyl groups, thereby forming a mixture of dibasic acids and trimellitic acid, thereafter subjecting said trimellitic acid to decarboxylation by heating at between about 150 and 400 C. in the presence of a substantially neutral aqueous reagent to produce a second mixture of dibasic acids consisting essentially of terephthalic acid and isophthalic acid, and recovering terephthalic acid and isophthalic acid from each of said two mixtures of dibasic acids.

9. A process as defined in claim 8 wherein said alkylation catalyst is essentially phosphorus pentoxide.

10. A process for producing terephthalic acid and isophthalic acid from a mixture of C-S aromatic hydrocarbons comprising by volume at least about 10% of ethylbenzene, at least about 20% of m-xylene, and a substantial proportion, over 5%, of p-xylene, which comprises subjecting said mixture to alkylation with propylene at 50 to 140 C. in the presence of an alkylation catalyst selected from the class consisting of phosphorus pentoxide, phosphoric acid, and sulfuric acid, continuing said alkylation for a time sufficient to effect substantially complete mono-alkylation of said ethylbenzene and any o-xylene present, terminating said alkylation (1) before any substantial amount of p-xylene has been Valkylated, (2) before any substantial amount of poly-alkylation has occurred, and (3) before complete mono-alkylation of said m-xylene has occurred, subjecting the resulting alkylation mixture to oxidation to convert substantially all of the alkyl side-chains to carboxyl groups thereby forming a mixture of dibasic acids and trimellitic acids, thereafter subjecting said trirnellitic acid to decarboxylation by heating at between about and 400 C. in the presence of a substantially neutral aqueous reagent to produce a second mixture of dibasic acids consisting essentially of te'rephthalic acid and isophthalic acid, withdrawing said second mixture of dibasic acids and separating terephthalic acid and isophthalic acid therefrom by fractional crystallization from water at successive temperature levels, first at between about 270 and 350 C. for recovery of solid terephthalic acid, and then at between about 20 and 150 C. for recovery of solid isophthalic acid.

11. A process as defined in claim 10 wherein said alkylation catalyst is essentially phosphorus pentoxide.

References Cited in the file of this patent UNlTED STATES PATENTS 1,551,373 Daudt Aug. 25, 1925 2,576,020 Knops Nov. 20, 1951 2,636,899 Burrows et al. Apr. 28, 1953 2,648,713 Schneider Aug. 11, 1953 2,734,914 McKinnis Feb. 14, 1956 OTHER REFERENCES Pittig et al.: Liebigs Annalen, vol. 148, p. 12 (1868). Nightingale et al.: I.A.C.S., Vol. 62, pp. 280-3 (1940). Welsh et al.: J.A.C.S., vol. 63, pp. 2603-4 (1941). 

1. A PROCESS FOR PRODUCING A MIXED ALKYLATE FROM A MIXTURE OF C-8 AROMTIC HYDROCARBONS COMPRISING BY VOLUME AT LEAST ABOUT 10% OF ETHYLBENZENE, AT LEAST ABOUT 20% OF M-XYLENE, AND A SUBSTANTIAL PROPORTION, OVER 5% OF P-XYLENE, SAID MIXED ALKYLATE BEING PARTICULARLY ADAPTED FOR TOTAL CONVERSION TO TEREPHTHALIC ACID AND ISOPHTHALIC ACID VIA OXIDATION AND DECARBOXYLATION, WHICH COMPRISES SUBJECTING SAID MIXTURE TO ALKYLATION WITH PROPYLENE, IN THE PRESENCE OF A CATALYST SELECTED FROM THE CLASS CONSISTING OF PHOSPHORUS PENTOXIDE, PHOSPHORIC ACID AND SULFURIC ACID, SAID ALKYLATION BEING EFFECTED AT A TEMPERATURE BETWEEN ABOUT 50* AND 140* C.,CONTINUING SAID ALKYLATION FOR A TIME SUFFICIENT TO EFFECT SUBSTANTIALLY COMPLETE MONOALKYLATION OF SAID ETHYLBENZENE AND ANY O-XYLENE PRESENT, AND TERMINATING SAID ALKYLATION (1) BEFORE ANY SUBSTANTIAL AMOUNT OF P-XYLENE HAS BEEN ALKYLATED, (2) BEFORE ANY SUBSTANTIAL AMOUNT OF POLYALKYLATION HAS OCCURRED, AND (3) BEFORE COMPLETE MONOALKYLATION OF SAID M-XYLENE HAS OCCURRED. 